1. Field of the Invention
The present invention relates to a microporous polyethylene film and a method of producing the same. More particularly, the present invention pertains to a microporous polyethylene film, which has a high productivity due to a superior extrusion-compoundability, and which can improve performance and stability of a battery produced using the same, and a method of producing the same.
2. Description of the Prior Art
Having chemical stability and superior physical properties, a microporous polyolefin film is widely used as various battery separators, filters, and ultrafiltration membranes.
The production of the microporous film using polyolefin may be conducted according to the following three processes. In a first process, polyolefin is processed into a thin fiber to produce a nonwoven fabric-shaped microporous film, a second process is a dry process, in which a thick polyolefin film is prepared and stretched at low temperatures to create micro cracks between lamellas corresponding to a crystalline portion of polyolefin to form micro pores in polyolefin, and a third process is a wet process, in which polyolefin is compounded with a diluent at high temperatures to form a single phase, phase separation of polyolefin and diluent is initiated in a cooling step, and the diluent is extracted to form pores in polyolefin. In comparison with the first and second processes, the wet process, corresponding to the third process, produces a relatively thin microporous film with uniform thickness and excellent physical properties, and thus, the microporous film according to the wet process is widely used for an isolation membrane of a secondary battery, such as a lithium ion battery.
A method of producing a porous film according to a wet process is disclosed in U.S. Pat. No. 4,247,498, which comprises blending polyethylene and a compatible liquid with each other at high temperatures to form a thermodynamically homogeneous solution, and cooling the solution to initiate solid/liquid or liquid/liquid phase separation, thereby producing the porous polyolefin film.
U.S. Pat. No. 4,335,193 discloses a technology of producing a porous polyolefin film, which includes blending polyolefin, an organic liquid, such as dioctylphthalate and liquid paraffin, and inorganic filler; forming the blend; and removing the organic liquid and inorganic filler from the formed blend, which is also indicated by U.S. Pat. No. 5,641,565. However, the technology is disadvantageous in that the inorganic filler, such as silica, is used in compounding process, it is difficult to conduct feeding and compounding processes of the inorganic filler, and a subsequent process of extracting and removing the inorganic filler must be additionally conducted, and thus, the technology becomes very complicated and also it is difficult to increase a stretching ratio.
U.S. Pat. No. 4,539,256 recites a basic method of producing a microporous film, which includes extrusion molding a mixture of polyethylene and a compatible liquid, stretching the formed mixture, and extracting the compatible liquid from the stretched mixture.
In conjunction with the earnest use of a secondary battery, efforts have been continuously made to improve the productivity and physical properties of a microporous film. A representative example is to improve the strength of the microporous film by using ultra-high molecular weight polyolefin (UHMWPO) with a weight average molecular weight of about 1,000,000, or blending such a UHMWPO with a composition to increase a molecular weight of the composition.
With respect to this, U.S. Pat. Nos. 4,588,633 and 4,873,034 suggest a process of producing a microporous film, in which polyolefin with a weight average molecular weight of 500,000 or more and a diluent capable of dissolving polyolefin at high temperatures are subjected to two step solvent extraction and stretching steps. However, this process is disadvantageous in that in order to improve a poor compoundability of UHMWPO with diluent and a poor extrudability of UHMWPO, which are considered as disadvantages of UHMWPO, an excessive amount of diluent is used in an extruding step, and diluent must be extracted through two steps, before and after stretching.
Meanwhile, Japanese Pat. Laid-Open Publication No. Hei. 03-245457 suggests a technology to enhance stability and reliability of a battery, in which two or more fine porous membranes, made of polyolefin, are attached to each other and one of the fine porous membranes is made of crosslinked polyolefin.
Furthermore, Japanese Pat. Laid-Open Publication No. Hei. 01-167344 provides a process of producing a microporous film, which includes adding a crosslinking agent and a crosslinking aid into an organic solvent solution to form a blend, and crosslinking the blend through an extrusion process. However, the microporous polyolefin film is disadvantageous in that it is not fit to be used as a secondary battery because of poor tensile strength of 330 kg/cm2 or less, it is difficult to control a viscosity of the blend in an extruder because polyethylene chains are bonded to each other due to a crosslink during an extrusion process, and it is difficult to produce a uniform gel-free film or sheet because of generation of gels.
U.S. Pat. No. 6,127,438 discloses a process of producing a microporous film, which includes forming a sheet made of polyethylene and a plasticizer, stretching the sheet, extracting the plasticizer, and irradiating the resulting sheet with an electron beam to crosslink the sheet, thereby increasing the strength of the microporous polyethylene film. However, this process is problematic in that since the process includes an additional electron beam irradiation step, safety is in question and production costs are undesirably increased.
Recently, there is a demand for a lithium ion battery, which assures a high capacity, excellent productivity and safety. In order to meet the demand, the prior arts as described above use a resin with a high molecular weight or adopt a crosslinking process to improve physical properties of a film and safety and reliability of the battery. However, use of the resin with the high molecular weight or addition of a crosslinking agent during an extrusion process may bring about problems, such as an increased extrusion load, a poor extrusion-compoundability of a resin with a diluent, an increased load of a stretcher during a stretching process, occurrence of non-uniform stretching, and a reduced productivity due to a decrease of a stretching speed and ratio, and may also lead to reduced safety and increased production costs due to use of radioactive substances in the case of crosslinking the film by irradiating the film with an electron beam after the film is formed.
The present inventors have conducted extensive studies to avoid the above disadvantages occurring in the prior arts, resulting in the finding that when peroxide is added to polyethylene with a low molecular weight and a mixture is then extruded, the molecular weight is increased during an extrusion process, thereby accomplishing the present invention.